1. Field of the Invention
The present invention relates to thin-film capacitors used for compensating for the dependence of the junction capacitance of a semiconductor element on temperature so that the temperature dependence of an electronic circuit using the semiconductor element is decreased.
2. Description of the Related Art
A thin-film capacitor generally has a structure formed of a bottom electrode on a substrate, a dielectric layer, and a top electrode laminated in this order and, in some cases, has a structure formed of a semiconductor substrate serving as a bottom electrode, a dielectric layer, and a top electrode layer laminated in this order.
In this type of thin-film capacitor, it has been desired that the relative dielectric constant and the Q factor of the dielectric layer be large and that the temperature coefficient of the resonant frequency be either a positive or negative value close to zero.
Heretofore, as a dielectric composition having the properties described above, the material described below has been known. This dielectric composition is formed by firing a BaOxe2x80x94TiO2-based dielectric material mixed with samarium oxide (Sm2O3), gadolinium oxide (Gd2O3), dysprosium oxide (Dy2O3), or europium oxide (Eu2O3). However, according to techniques for obtaining this type of dielectric ceramic composition, the relative dielectric constant xcex5xc2x7r and the temperature coefficient xcfx84 can only be controlled in the range of from 61 to 72 and in the range of from xe2x88x9224 to 31 ppm/xc2x0 C., respectively.
Technical development has been conducted in view of the situation described above, and as a result, a dielectric ceramic composition is provided having a laminated structure formed by adhering a first dielectric ceramic composition sheet having a positive temperature coefficient of the resonant frequency to a second dielectric ceramic composition sheet having a negative temperature coefficient of the resonant frequency.
According to the technique described above, a mixture of starting materials used for forming a desired composition is molded so as to form a cylindrical body 16 mm in diameter and 9 mm thick, and this cylindrical body is fired at 1,260 to 1,450xc2x0 C. for several hours, thereby forming the first dielectric composition. In addition, a mixture of starting materials different from that described above is molded and is then fired, thereby forming the second dielectric ceramic composition having the same dimensions as those described above. The two dielectric ceramic compositions thus formed are cut into sheets having a thickness of approximately 1 mm, and the sheets thus obtained are laminated to each other, so that a laminated dielectric ceramic composition is obtained.
In more particular, a desired relative dielectric constant and a desired temperature coefficient can be obtained by laminating dielectric ceramic compositions having the same relative dielectric constant or different relative dielectric constants from each other and by adjusting the volume ratio of the dielectric ceramic compositions described above.
According to the technique described above, since the structure is formed by laminating a plurality of sheets of the first dielectric ceramic composition and the second dielectric composition, each having a thickness of approximately 1 mm or less, the structure can be applied to a sheet-shaped laminated capacitor; however, it has been difficult to further miniaturize the capacitor or reduce the weight thereof. For example, a capacitor having a thickness of 1 mm or less cannot be formed, and hence, it has been difficult to form an even thinner capacitor.
In addition, when the sheets of the dielectric ceramic compositions are laminated to each other by using an adhesive, since the adhesive layer or an air layer having different dielectric properties is present at the interface between the sheets, a plurality of discontinuous portions is formed in the thickness direction of the laminated sheet structure. Consequently, it has been difficult to obtain a desired dielectric material having an ideal temperature coefficient.
Furthermore, since the dielectric ceramic composition in the form of a sheet is composed of a thick polycrystalline dielectric material, a number of crystalline grain boundaries are present in the thickness direction thereof, and as a result, it has been difficult to decrease the dielectric loss in a high frequency band of 1 GHz or more.
In view of the situations described above, the present invention was made, and an object of the present invention is to provide a thin-film capacitor which can easily satisfy the demand for a reduction in size, thickness, and weight, and which can also perform temperature compensation. In addition, another object of the present invention is to provide a thin-film capacitor having a superior Q factor in a high frequency band in addition to the features described above of the present invention. Furthermore, another object of the present invention is to provide a thin-film capacitor having a small leakage current.
In order to solve the problems described above, a thin-film capacitor according to the present invention comprises at least one first dielectric thin-film, at least one second dielectric thin-film having a relative dielectric constant different from that of the first dielectric thin-film, and a pair of electrodes, wherein the first thin-film and the second dielectric thin-film are provided between the pair of electrodes.
Since the said at least one first dielectric thin-film and the said at least one second dielectric thin-film having a relative dielectric constant different from that of the first dielectric thin-film are disposed between the pair of electrodes, by the combination of the dielectric thin-films described above, adjustment of the Q factor, adjustment of the withstand voltage, and the temperature compensation can be performed.
In the temperature compensating thin-film capacitor according to the present invention, it is preferable that the absolute value of the temperature coefficient of capacitance of the first dielectric thin-film be 50 ppm/xc2x0 C. or less, and that the temperature coefficient of capacitance of the second dielectric thin-film be negative and have an absolute value of 500 ppm/xc2x0 C. or more.
Since the dielectric thin-films each having the temperature coefficient of capacitance in the range described above are laminated to each other, the temperature coefficient can be adjusted, and the temperature compensation can be performed.
In the structure of the temperature compensating thin-film capacitor of the present invention, the second dielectric thin-film comprises a polycrystalline material having a number of crystalline grains forming a number of crystalline grains boundaries, and the number of the crystalline grain boundaries is preferably less than ten in the thickness direction of the second dielectric thin-film.
When the second dielectric thin-film has less than ten crystalline grain boundaries in the thickness direction thereof, and more preferably, when the second dielectric thin-film does not have a plurality of (for example, at least two) crystalline grain boundaries, a low dielectric loss in a high frequency band can be achieved, and a Q factor in a high frequency band can be increased.
In the structure of the present invention described above, the first dielectric thin-film preferably has a relative dielectric constant of 10 or less; a breakdown field strength (an electric field strength at which the current density is abruptly increased) of 5 MV/cm or more, and more preferably, 8 MV/cm or more; a Q factor of 200 or more, and more preferably, 500 or more at a frequency of 1 GHz or more; and a dielectric relaxation time of 1 second or more. Consequently, a thin-film capacitor having a smaller thickness and a higher withstand voltage can be obtained, and in addition, a thin-film capacitor suitably used for a high-frequency circuit can also be obtained.
In the structure of the present invention described above, the second dielectric thin-film preferably has a relative dielectric constant of 150 or less; and a Q factor of 50 or more, and more preferably, 100 or more at a frequency of 1 GHz or more.
In the structure of the present invention described above, the first dielectric thin-film preferably comprises SiOxNy.
In the structure of the present invention described above, the second dielectric thin-film preferably comprises one of TiOx and CaTiO3.
When the first dielectric thin-film comprises SiOxNy, a superior withstand voltage and a high Q factor can be easily obtained, and when the second dielectric thin-film comprises TiOx or CaTiO3, the temperature coefficient of capacitance can be easily adjusted by controlling the thickness thereof.
In the structure of the present invention described above, as the dielectric thin-films disposed between the pair of electrodes, the first dielectric thin-films are each preferably provided on each electrode, and the second dielectric thin-film is preferably provided between the first dielectric thin-films.
When the first dielectric thin-films each comprises SiOxNy and are each provided on each electrode, the withstand voltage can be increased, and the current leakage can be reduced. In addition, when the second dielectric thin-film is provided between the pair of first dielectric thin-films, the temperature coefficient can be easily controlled.